1. Field of the Invention
The present invention relates to a method of processing an image signal, and more particularly to a method of converting an image signal such as R (red), G (green), B (blue) signals generated by an imaging device such as a color digital camera (also referred to as a digital camera) and representing an original scene which is captured by the imaging device, or an imaging signal such as R, G, B signals generated by an image reading device such as a color scanner (also referred to as a scanner), into a colorimetric signal, processing a colorimetric signal for setup, converting a calorimetric signal processed for setup into a dye density signal for use on a reversal medium, or converting a colorimetric signal processed for setup into c, m, y, k (cyan, magenta, yellow, and black) signals for being supplied to an image output device such as a film producing device, a plate producing device, a color printer, or the like.
2. Description of the Related Art
Device-dependent image signals such as R, G, B signals generated by color digital cameras are converted into device-independent image signals such as tristimulus signals X, Y, Z according to color conversion processes disclosed in Japanese laid-open patent publications Nos. 2-291777 and 2-291778, for example.
Techniques for predicting reproducible colors on prints, for example, device-independent tristimulus signals X, Y, Z from device-dependent c, m, y halftone dot % signals or reproducible colors based on color signals c, m, y, k are disclosed in Japanese laid-open patent publications Nos. 4-337965 and 4-337966, for example.
According to a process of accurately converting colors as disclosed in the Journal of the Institute of Image Electron, Vol. 18, No. 5 (1989), a three-dimensional (XYZ) color space is divided into 512 cubic regions, and color correcting values at a total of 729 grid points are calculated in a process optimized for the characteristics of an output device and stored as the data of a look up table (LUT). Input values between the grid points are determined by a interpolating process which three-dimensionally interpolates the LUT.
According to another known process, R, G, B signals produced from a color reversal subject (prepared by exposing a color reversal film to light from an image, developing the image on the color reversal film) which carries a positive image are converted into equivalent neutral density (END) signals by a color scanner which is a linear scanning reader, and the END signals are converted into halftone dot % signals with reference to setup points (highlight and shadow points) determined by a setup process. The halftone dot % signals are then converted into c, m, y, k signals, which are then binarized (i.e., converted into halftone dots), finally producing a printing plate or a printed material.
With conventional color scanners, R, G, B signals are processed for setup based on color-separating default conditions (image processing conditions including at least a gradation conversion process, a color correction process, an under color removal process, and a K-plate generating process) which are carried on the respective color scanners and empirically optimized, and R, G, B signals produced from a color reversal subject are converted into c, m, y, k signals.
With respect to R, G, B signals produced by a digital camera, color-separating default conditions may be established using an image processing tool (software) such as Adobe Photoshop (registered trademark).
It is a major object of the present invention to provide a method of processing an image signal, which can easily convert device-dependent image signals into device-independent image signals.
Another object of the present invention is to provide a method of processing an image signal, which allows a setup process based on END signals, for which gray conditions are prescribed in a device-dependent signal system, to be applied to a device-independent signal system.
Still another object of the present invention is to provide a method of processing an image signal for producing c, m, y, k signals capable of reproducing colors of an original scene with ease and accuracy.
Yet still another object of the present invention is to provide a method of processing an image signal, which is capable of converting image signals of an original scene captured by a digital camera into color signals that can utilize empirically optimized standard default separating conditions of an existing scanner.
A further object of the present invention is to provide a method of processing an image signal, which will convert R, G, B image signals generated by a digital camera or R, G, B image signals read by a scanner into c, m, y, k signals using empirically optimized default separating conditions of a particular existing scanner.
The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example.